The overall goal of this study is to study the context and epidemiology of HIV, STIs and associated risk behaviors among high risk female sex workers (FSWs) and their non-commercial male partners in two Mexico- US border cities. Preliminary data collected by our binational team over the past 5 years suggest that FSWs' non-commercial partners may be significant drivers of HIV/STI acquisition and/or their re-infection, which is supported by literature in other countries. FSWs in our setting are more than twice as likely to engage in unprotected sex with their main non-commercial partner; half of these partners have concurrent partnerships and one third are IDUs. The proposed study of FSW-partner pairs (dyads) builds on our existing research infrastructure in Tijuana and Cd. Juarez to meet the following aims: Aim 1: To examine the a) context and b) patterns of high risk sexual and substance using behaviors among high risk FSWs and their main non- commercial male partners, both within and outside of the partnerships, using a mixed-methods approach; Aim 2: To determine prevalence of HIV and specific STIs (i.e., syphilis, gonorrhea, Chlamydia, HSV-2) and associated correlates at the individual and partner level; Aim 3: To prospectively identify predictors of STI incidence and their attributable risks at the individual and partner level; Aim 4: To determine the feasibility of conducting a behavioral intervention trial among high risk FSWs and their main non-commercial male partner at the a) partner level and b) individual level, using mixed methods. To meet these aims, we will recruit 100 FSW-partner dyads in both Tijuana and Cd. Juarez (i.e., 200 couples), who will undergo interviews and HIV/STI testing at baseline and follow-up for 24 months to address both clinical and behavioral outcomes (Aims 2 and 3). A subset (25 couples per city) will undergo in-depth interviews at baseline and once during follow-up to refine the quantitative survey, and inform a mixed methods analysis to address Aims 1 and 4. Using qualitative and quantitative methods, we will assess each partner's interest in participating in a subsequent intervention, attrition and partnership dissolution, and potential barriers to interventions such as intimate partner violence (Aim 4). To our knowledge, the proposed study is the first prospective evaluation of FSWs and their non-commercial partners. Our binational research team is uniquely positioned to conduct this study, which will inform the response to the growing HIV crisis on the Mexico-US border and efforts to curtail HIV transmission among FSWs and their partners in other resource-constrained settings.